1. Field of the invention
The present invention relates to a method for locking a probe of a scanning probe microscope to any desired position on a specimen surface for any desired time period.
2. Related Art Statement
Since a scanning probe microscope, for example a scanning tunneling microscope (STM) can investigate a fine surface structure of solid bodies and can obtain useful information about various characteristics of various materials with an atomic resolution, the scanning probe microscope has been widely used as an essential measuring and analyzing tool not only in the basic research but also in the application research.
The scanning probe microscope utilizes an interaction between a probe and a specimen, said interaction spreading on a two dimensional specimen surface plane over a distance substantially equal to a dimension of one atom. Therefore, by moving the probe into any desired position on the specimen surface plane, information at this position can be obtained.
Usually, the probe secured to a piezo electric element can be moved by applying a voltage to the piezo electric element. Therefore, if the voltage is kept constant, a position of a tip of the probe relative to the specimen surface plane does not change. However, in practice, it is known that a relative position between the probe and the specimen varies in accordance with time. This phenomenon has been known as a thermal drift of the probe. Such a probe drift has been known from the development of the scanning probe microscope. In various applications and research, it is required to lock the probe with respect to the specimen for a relatively long time, but due to the probe drift, the probe could not be fixed or locked to a desired position for a long time. For instance, in the STM light emission spectroscopy in which STM images are analyzed in relation to light emission spectrum, since emitted light is very weak, a quite long time period such as several minutes is required for the analysis. However, the probe tip could not be locked at a desired point for such a long time period.
There have been proposed several solutions for resolving the above mentioned probe drift. For instance, an amount of the probe drift is predicted on the basis of a previously measured probe drift and a driving voltage applied to the piezoelectric element is adjusted on the basis of the predicted amount of the probe drift. However, in practice, since the probe drift does not occur uniformly and an actual probe drift could not be predicted correctly, this known method could not correct the probe drift sufficiently. It is quite likely that the probe drift during the previous measurement differs from the probe drift during an actual inspection.
There has been also proposed another known method for locking the probe. This method is called a tracking tunneling microscopy (TTM) belonging to the STM. A principle of this method will be described with reference to FIG. 1.
FIG. 1 shows diagrammatically the principle of the known TTM. As shown in FIG. 1, according to the TTM, the probe is moved circularly on a specimen surface plane while a tunnel current is measured. During the circular movement of the probe, when the probe traces a locally inclined portion of a specimen, the tunnel current varies because a distance from the probe tip and the actual specimen surface is changed. This variation in the tunnel current is fedback to the driving voltage to be applied to the piezoelectric element such that the probe is moved along the inclined portion of the specimen surface. Finally, the probe reaches a flat portion of the specimen surface, i.e. a top or bottom of the crenellated structure of the specimen surface and is locked at this flat portion, because the tunnel current does not change any more at this flat portion.
It is known that this method can lock the probe with an atomic resolution, but an area at which the probe can be locked is limited to the top or bottom of the crenellated specimen surface structure. Namely, the probe could not be locked at any desired position on the specimen surface plane.
As explained above, there has not been proposed a practically useful method for solving the problem of the probe drift effectively.